Stator of ac electric machine

ABSTRACT

A stator with a magnetic circuit having separate slots to accommodate coil groups of a three-phase delta- and star-connected windings. Vectors of magnetic induction of each coil group intersect an axis of the magnetic circuit. The coil groups of like phases overlap one another in a cross section of the stator and are displaced through 30 electrical degrees. The three-phase windings are made with similar power ratings. This provides substantial temporal and geometric orthogonality between pairs of vectors of one phase of one three-phase winding and the coil groups of the next phase of the other three-phase winding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to ac electric machines and more specifically to astator an ac electric machine.

2. The Prior Art

Widely known in the prior art at present is a stator or an ac electricmachine (EP 0271604), having a cylindrical magnetic circuit whichoccupies a set phase zone. The slots of the machine internallyaccommodate coil groups of two three-phase windings which are star anddelta-connected and provided with three leads designed for connection toan external mains.

Previously, a star-connected three-phase winding is a main winding,while a delta-connected three-phase winding is an auxiliary winding witha substantially-reduced power (of up to two orders of magnitude). Bothwindings are connected in parallel to a supply source and areaccommodated in common slots of the magnetic circuit. The coil groups oflike phases of the three-phase windings are displaced relative to oneanother in the cross section of the stator through 90 electrical degreesand their corresponding leads are connected to different bus bars of athree-phase mains, where the time-phase displacement comprises 120°.

The embodiment and disposition of the three-phase windings describedhereinbefore is intended to compensate for odd harmonics of anelectromagnetic field of the electric machine stator. From thisstandpoint the characteristics of the stator electromagnetic field areupgraded, however this known design solution has no effect on theconversion of electrical energy into mechanical energy (operation inmotor mode) and vice versa (operation in generator mode). In addition,there is no effect on the function of an electric machine as determinedby speed-torque characteristics, torque ratio, current ratio, etc.

It is know that the electromagnetic force F in an ac electric machineapplied to bus bars of the rotor depends on a magnitude of the vector ofcurrent I passing through the rotor bus bar and induced according to theelectromagnetic induction law. The vector is induced by the magneticflux of one phase of the stator on a magnitude of the vector of themagnetic induction B of the magnetic field of another phase of thestator, as well as on a geometric angle between the vectors whichfollows from Ampere's formula:

    F=B×I×1 sin ,

where 1 is the active length of the rotor bus bar.

Thus, the optimum conditions for conversion of electrical energy intomechanical energy and vice versa will take place with the availabilityof geometric orthogonality and phase coincidence of vectors of the rotorcurrent and stator magnetic induction.

At the same time, it is known that according to the electromagneticinduction law the rotor current lags from the magnetic field whichinduced the latter through a phase angle π/2. From this it follows thatthe optimum conversion of energy as applied to the stator of an electricmachine described hereinbefore will take place in case when thegeometric and phase angles between the vectors of magnetic induction ofthe adjacent phases of the stator will be equal to π/2. In other wordsgeometric and phase orthogonality will provide optimum energyconversion.

In a known stator, the geometric and phase angles between said vectorsof the magnetic induction comprise 90° and 120°, respectively. Due tosuch relative orientation of the magnetic induction vectors the latter,in the general case, do not intersect the axis of a magnetic circuit.This leads to the fact that the optimum conditions for conversion ofelectrical energy into mechanical energy and vice versa are providedonly in a narrow speed range (close to a synchronous speed or when anelectric machine operates with a small slip). In addition, the use ofsaid stator in an electric machine, particularly in an electric motor,prevents parametric control of the motor without changing the frequencyof supply current of voltage.

As a rule, electric motors are designed for a rated mode of operation inwhich they have a high efficiency from 80% to 90%. Practically, such amode is comparatively rare in practice and in the case of frequentstarts of the motor, variations of voltage in the supply mains, andperiodic operation at no-load. The actual efficiency of the electricmotor turns out to be low, around 6-17°. It should be noted that theelectric motor with a known stator has a statistically unsteadyspeed-torque characteristic throughout 80% of the speed range and as acommon induction motor, it has a torque ratio close to a unit and acurrent ratio of 6 to 7.5.

The known stator is characterized by a low manufacturability as the sameslots of the magnetic circuit serve for accommodating the coils of bothwindings which ave conductors of different diameters. Such a process islabor intensive and makes it practically impossible to automate theassembly process.

SUMMARY OF THE INVENTION

The present invention provides a stator of an ac electric machine withsuch arrangement, of coil groups of the three-phase windings in theslots of a magnetic circuit. The relative position of the coil groups oflike phases of the three-phase windings and selection of their powerallows conversion of electrical energy into mechanical energy and viceversa in a mode close to the optimum. The conversion occurs over a widespeed range which in turn will upgrade the energy indexes of an acelectric machine with the proposed stator, will provide a parametriccontrol of the rotor speed across a wide speed range without changingthe current frequency and will improve the manufacturability of thestator.

These objects are attained by a stator of an ac electric machinecomprising a cylindrical magnetic circuit, the slots of which aredisposed in a set phase zone. The slots accommodate coil groups of twothree-phase delta- and star-connected windings, and are provided withthree leads for connection to an external circuit. According to theinvention, the coil groups of each three-phase winding are accommodatedin separate slots of the magnetic circuit so that the vector of themagnetic induction of the magnetic field induced by the current flowingthrough each of the coil groups intersects the axis of the magneticcircuit. In addition, the coil groups of like phases of the three-phasewindings overlap one another in the cross section of the stator and aresubstantially displaced through 30 electric degrees. The three-phasewindings are manufactured close in power which in total, substantiallyprovides the time and geometric orthongonality between the pairs ofvectors of the magnetic induction of the magnetic fields set up by thecurrent flowing through the coil groups of one phase of thedelta-connected three-phase winding and the coil groups of a next phaseof the star-connected three-phase winding.

To provide the possibility for independent connection of the three-phasewindings, it is expedient that the three-phase windings by provided withthree additional leads so that each three-phase winding is provided withindividual leads. Structurally, it is advantageous that the coil groupsof the three-phase windings be disposed in separate slots of themagnetic circuit so that the phase zone occupied by each coil group inthe cross section of the stator is a multiple of 30 within 60 to 180electrical degrees, except for 150 electric degrees.

The proposed stator makes it possible to convert electric energy intomechanical energy and vice versa in a mode close to the optimum due tofulfillment of the required conditions. If an electric motor with awidely used short-circuited rotor is provided with the proposed stator,then said motor becomes controllable by the amplitude of voltage, withthe frequency of the supply current being unchanged. The speed-torquecharacteristic of such an electric motor becomes statically steadyacross the entire speed range and may have a "soft" or "excavator" form.

Besides the power of an electric motor provided with the proposed statoris stepped up by 30-40% in comparison with an electric motor having theknown stator. Also, the mass and heat loads of the proposed statorremain the same. At the same time, the torque ratio is stepped up by2.5-3 times, that is the starting torque is increased by 2.5-3 times incomparison with the known electric motor. The current ratio is decreasedby 2.9-3.2 times, which improves the operational reliability of theelectric motor by 2-3 times.

The use of independent leads of the three-phase windings makes itpossible to individually control each of the three-phase windings. Thisin turn allows the speed-torque characteristic of a plurality ofspecial-purpose electric machines to be adapted to a driven electricmachine in the process of operation in compliance with a varyingcharacteristic of the driven machine. Noteworthy also is a highmanufacturability of the proposed stators allowing the process ofassembly thereof to be automated which is of particular importance forelectric machines with a number of poles 2p=2.

A very important advantage of the proposed stator resides in operatingin a mode close to the idling current of the rated mode at a voltagecomprising about 30% of the rated value (known electric machines arebrought in a rated mode at a voltage differing from the rated value bynot more than 5%). This fact leads to increase in a mean value of theefficiency with regard to frequent starts, variations in voltage of thesupply mains, no-load operation and other factors. It should be notedthat all the advantages listed hereinbefore are applicable to electricmachines and are also inherent to electric generators provided with theproposed stator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tospecific embodiments thereof, taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 illustrates a connection diagram of coil groups of three-phasewindings of an ac electric machine stator, according to the invention;

FIG. 2 illustrates a vector diagram of magnetic induction of coil groupsof the three-phase winding connected according to FIG. 1;

FIG. 3 is a fragment of the diagram illustrating the layout of coilgroups of three-phase windings in slots of the magnetic circuit,according to the invention; and

FIG. 4 is a time diagram of currents induced in three-phase windings ofan ac electric machine stator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a stator of an ac electric machine including a cylindricalmagnetic circuit 1 (FIG. 1) whose slots (not shown in the drawing andthe magnetic circuit is illustrated arbitrarily) internally accommodate,along the circumference, coil groups 2, 3 and 4 of a delta-connectedthree-phase winding and coil groups 2', 3' and 4' of a star-connectedthree-phase winding. The three-phase windings are illustrated in FIG. 1arbitrarily in a cross section of the stator, and in the hereindescribed alternate embodiment the coil groups 2, 3 and 4 encompass thecoil groups 2', 3' and 4' however, it will be apparent that said coilgroups may be arranged in any other manner. Each coil group 2, 3 and 4;2', 3' and 4' of the herein described alternative embodiment comprisestwo components (coils) symmetrically spaced along the circumference.Designations U1, V1, and W1 correspond to starting points of adelta-connected three-phase winding and designations U2, V2 and W2correspond to ending points of said phases. Analogous designations areused for defining starting and ending points of a star-connectedthree-phase winding: 1U1, 1V1 and 1W1 are the starting points and 1U2,1V2 and 1W2 are the ending points, respectively, and N is a neutralpoint (zero).

Referring to FIG. 1, there are shown continuous lines illustratingvectors of magnetic induction BU1V1, BW1U1, BV1W1 of magnetic fields setup by currents passing through the coil groups 2, 3 and 4, respectively.Analogously, dash lines are used for illustrating vectors of magneticinduction B1U1, B1W1, B1V1 of magnetic fields set up by currents passingthrough the coil groups 2', 3' and 4' of the star-connected three-phasewinding.

The coil groups 2, 3 and 4; 2', 3' and 4' are disposed in the slots ofthe magnetic circuit 1 so that all the given vectors of the magneticinduction intersect an axis 5 of the magnetic circuit 1 which is one ofthe hereinbefore described conditions required for an optimum mode ofthe electric machine operation. The coil groups 2--2'; 3--3'; 4--4' oflike phases of the three-phase windings overlap one another and aresubstantially displaced through 30 electrical degrees (an angle of 30°between the starting points of the coil groups 2--2' is illustrated asan example in the drawing which is valid and evident for other coilgroups 3--3'; 4--4').

A necessity for displacement between the coil groups 2--2'; 3--3'; 4--4'of the like phases of the three-phase windings through an angle of 30electrical degrees follows from the hereinbefore described analysis ofthe conditions required for an optimum conversion of electric energyinto mechanical energy and vice versa. As the angle between the coilgroups 2, 3 and 4; 2', 3' and 4' of one phase in each winding amounts to120°, the angle between the vectors of magnetic induction of the coilgroups 3, 4 and 2; 3', 4' and 2' of next phases should amount to 90electrical degrees. In order to fulfil this condition the coil groups2--2'; 3--3'; 4--4' of the like phases should be displaced through 30electrical degrees. However, it is necessary to take into account thatthe high-energy parameters of an electric machine may be provided onlyif the phase zone occupied by one coil group 2, 3 and 4; 2', 3' and 4'is over 30 electrical degrees. Thus, the coil groups 2--2'; 3--3'; 4--4'of the like phases of windings with regard to their displacement through30 electrical degrees should by all means overlap one another.

Such an arrangement of the three-phase windings in the slots of themagnetic circuit 1 makes it possible to achieve a substantiallygeometric orthogonality between the pairs of vectors of magneticinduction of the magnetic fields set up by currents passing through thecoil groups 2, 3 and 4 of one phase of the delta-connected three-phasewinding and the coil groups 3', 4' and 2' of the next phase of thestar-connected three-phase winding. So in the herein describedalternative embodiment the orthogonality is observed between thefollowing pairs of the magnetic induction vectors: BU1V1 (coil group 2)and B1W1 (coil group 3'); BW1U1 (coil group 3) and B1V1 (coil group 4');BV1W1 (coil group 4) and B1U1 (coil group 2').

In the herein describer alternate embodiment of the invention, eachthree-phase winding has individual leads which make it possible toaccomplish a parallel or an independent connection to an externalcircuit (supply source or load). So. the starting points U1, V1, W1,1U1, 1V1, 1W1 of respective phases of the windings serve as leads of thedelta-connected winding and as leads of the star-connected winding.

An obvious advantage of the herein described construction resides inthat an independent three-phase supply source (load) may be connected toeach of the three-phase windings which allows the parametric control tobe effected both as a combined (joint) control and a separate(individual) control. This makes it possible to control the form of thespeed-torque characteristics which may be in a "soft" or an "excavator"form statistically steady throughout the entire speed range. Due to thefact that the speed-torque characteristics are widely known, they arenot illustrated in the drawing.

Referring now to FIG. 2, there is shown the vector diagram of magneticinduction of the coil groups 2, 3 and 4; 2', 3' and 4' of thethree-phase windings connected in the manner described hereinbefore. Thethree-phase windings are made close in power (design) which makes itpossible to achieve the phase angle close to π/2. In other words, it ispossible to achieve a time orthogonality between said pairs of vectorsof the magnetic induction: BU1V1 and B1W1; BW1U1 and B1U1; BV1W1 andB1V1.

Achieving geometric and temporal orthogonality between the vectors ofmagnetic induction provides the required conditions for an optimumconversion of electrical energy into mechanical energy and vice versa.It should be noted that space relations may be effected with an errordepending upon design and technological causes.

It should also be noted that there is a one-to-one correspondencebetween the direction of the vector of magnetic induction of the coilgroup 2, 3 and 4; 2', 3' and 4' of the phase winding or of the stator asa whole. There is also one-to-one correspondence in the direction of thestator current or EMF on the one hand, and the arrangement of coilgroups 2, 3 and 4; 2', 3' and 4' and the phase relations of currentstherein on the other hand. Therefore, the arrangement of the coil groups2, 3 and 4; 2', 3' and 4' may be assigned by the direction of theinduction vector and vice versa. In other words, the coil groups 2, 3and 4; 2', 3' and 4' are arranged and connected to an external circuitso that the phase and space relations described hereinbefore areobserved.

In the alternative embodiment of the invention, the phase zone occupiedby each coil group 2, 3 and 4; 2', 3' and 4' comprises 60 electricaldegrees (in the drawing the phase zone is marked as an example for thecoil group 3').

In other alternate embodiments of the invention, the magnitude of thephase zone may amount to 90, 120 or 180 electrical degrees. Selection ofthe phase zone magnitude is fully determined by the requirements to bemet by the electric machine. When the magnitude of a phase zone amountsto 60 or 90 electrical degrees, it is possible to cut down the weight ofa machine and consumption of copper for windings, and for electricmachines having the number of pairs of poles 2p=(3000 rpm) it is alsopossible to automate the process of laying windings. When the magnitudeof the phase zone amounts to 120 or 180 electrical degrees the energyindexes of an electric machine are stepped up. A fragment of the diagramillustrating the layout of the coil groups 2--2'; 3--3' of thethree-phase windings into the slots 6 (FIG. 3) of the magnetic circuit 1shows that conductors of the coil groups 2--2'; 3--3' are laid in theseparate slots 6. For convenience the conductors of the coil groups 2and 3 of the delta-connected winding are illustrated in the drawing bycontinuous lines, while the conductors of the coil groups 2' and 3' ofthe star-connected winding are illustrated by dash lines. Referencenumbers from 1 to 15 shown on a section of the magnetic circuit 1 are incompliance with the accepted numbering of the slots 6 of the magneticcircuit 1 of an electric machine stator.

Shown as an alternative embodiment of the invention is the winding of anelectric machine having 36 slots in which each slot 6 occupies 10electrical degrees. From this it follows that the phase zone occupied byeach coil group 2 and 2'; 3 and 3' comprises 90 electrical degrees. Suchan electric machine has the following number pairs of poles 2p=2.Referring now to FIG. 3, there is shown an alternative embodiment of adouble-layer winding, however it will be apparent that in otheralternative embodiments the winding may be of a single-layer or one anda half-layer type depending on the requirements to be me by electricalmachines. Referring to FIG. 4, there is shown a time diagram of currentscorresponding to magnetic fluxes in the stator three-phase windings.Curve a corresponds to current of the stator of a known machine, whilecurves b and c correspond to currents in the three-phase windings of theproposed stator. Curve d is a resultant of the curves b and c.

The proposed stator of an ac electric machine running in a motor modeoperates in the following manner. Voltage of the three-phase mains isapplied to the leads U1, V1, W1, 1U1, 1V1, 1W1 (FIGS. 1,3) of thewindings, in bus bars of the motor rotor (not shown in the drawing).Induced currents cooperate with magnetic fields set up by the currentspassing through the coil groups 2, 3 and 4; 2', 3' and 4' of therespective phases of the three-phase windings laid in the slots 6 ofmagnetic circuit 1.

As a result, an electromagnetic force is applied to bus bars of therotor and directed tangentially to the surface thereof, which forcedevelops the torque of the motor. When the proposed stator is used themagnitude of the electromagnetic force will be close to a maximum due tothe design features the arrangement of the three-phase windings whereinthe electric motor operates under conditions close to the optimum inconversion of electrical energy into mechanical energy.

The advantages of the proposed stator are illustrated by FIG. 4presenting time diagrams of currents induced in the known stator and inthe proposed stator. In the known stator, the amplitude of magneticinduction corresponding to the amplitude of current I_(ma) (curve a) islimited by a saturation induction of the electrotechnical steel of whichthe magnetic circuit is manufactured. In the proposed stator, the motortorque is governed by a total magnitude of current I_(m)Σ (curve d)obtained by addition of curves of the current in the three-phasewindings displaced in phase through π/2 (curves b and c). It is seenfrom the graph that the amplitude I_(m)Σ is by 30-40% smaller thanI_(ma), respectively and the amplitude of the total magnetic flux willbe by 30-40% smaller than that of the known stator.

As a result, the working point on the magnetization curve ofelectrotechnical steel (it is not given in the drawing, as it is wellknown) is achieved in an electric machine having a substantially smallermass and the same heat loads or in a machine having the same mass andoverall dimensions where the power rating may be increased by 30-40%. Atthe same time, the torque ratio increases by 2.5-3 times and the currentratio decreases by 2.9-3.2 times which improves operating reliability ofthe electric machine with the proposed stator by 2-3 times.

When the stator is used in an electric generator, the torque isconverted into appropriate three-phase voltages of the windings,provided the optimum conditions for conversion of mechanical energy intoelectric energy are duly observed. It should also be noted that theproposed stator steps up the mean value of efficiency with regard tovariations in the load, frequent starts and stops, and periodic idleperiods. This also makes it possible to control the rotor speed by thevoltage amplitude with the frequency of the supply current beingconstant. The proposed stator is characterized by high manufacturabilityallowing automation of the assembly process.

The present invention may be used to advantage in production of acmotors and generators, in construction of drives with parametric controlby voltage amplitude and with unchanged frequency of supply current, aswell as in systems and installations requiring control of the rotationalspeed, operating in start-and-stop modes and under conditions of varyingload and supply mains voltage.

The electric machines with the proposed stators may find wideapplication in mechanical engineering, oil and gas industry, in mining,and in manufacturing, light, food and other industries.

We claim:
 1. A stator of an ac electric machine electrically coupled toan external circuit comprising:a cylindrical magnetic circuit having acircumference and a plurality of slots spaced apart equidistantly alongthe circumference; a delta-connected three-phase winding comprisingthree coil groups arranged symmetrically along the circumference anddisposed within a first group of slots of said magnetic circuit tooccupy a set phase zone and generate magnetic fields when a currentpasses therethrough, said magnetic fields being represented by magneticinduction vectors BU1V1, BV1W1, and BW1U1; and a star-connectedthree-phase winding comprising three coil groups arranged symmetricallyalong the circumference and disposed within a second group of slots ofsaid magnetic circuit, distinct from said first group of slots, tooccupy a set phase zone and generate magnetic fields being representedby magnetic induction vectors B1U1, B1V1, and B1W1, said delta-connectedand star-connected three-phase windings having power ratings close toeach other; each three-phase winding having three leads adapted forconnection to the external circuit, coil groups from each three-phasewinding of like phases overlap one another in a cross section of thestator and are displaced 30 electrical degrees from each other, whereintemporal and geometric orthogonality occurs between pairs of magneticinduction vectors BU1V1-B1W1; BW1U1-B1V1; BV1W1-B1U1 generated bycurrents passing along coil groups of one phase of said delta-connectedwinding and coil groups of a next phase of said star-connected winding.2. The stator according to claim 1, wherein each three-phase windingincludes three additional leads adapted for connection to the externalcircuit so that each three-phase winding has a separate lead forindependent connection of said winding to the external circuit.
 3. Thestator according to claim 1, wherein said coil groups are disposedwithin separate slots of said magnetic circuit so that in the crosssection of the stator the phase zone occupied by each coil group is amultiple of 30 within a range from 60 to 180 electrical degrees, withthe exception of 150 electrical degrees.
 4. A stator of an ac electricmachine electrically coupled to an external circuit comprising:acylindrical magnetic circuit having a circumference and a plurality ofslots spaced apart equidistantly along the circumference; adelta-connected three-phase winding comprising three coil groupsarranged symmetrically along a circumference and disposed within a firstgroup of slots of said magnetic circuit to occupy a set phase zone andgenerate magnetic fields when a current passes therethrough, saidmagnetic fields being represented by magnetic induction vectors BU1V1,BV1W1, and BW1U1; and a star-connected three-phase winding comprisingthree coil groups arranged symmetrically along a circumference anddisposed within a second group of slots of said magnetic circuit,distinct from said first group of slots, to occupy a set phase zone andgenerate magnetic fields being represented by magnetic induction vectorsB1U1, B1V1, and B1W1, said three-phase windings having power ratingsclose to each other; each three-phase winding having three leads adaptedfor connection to the external circuit, coil groups from eachthree-phase winding of like phases overlap one another in a crosssection of the stator and are displaced 30 electrical degrees from eachother, wherein temporal and geometric orthogonality occurs between pairsof magnetic induction vectors BU1V1-B1W1; BW1U1-B1V1; BV1W1-B1U1generated by currents passing along coil groups of one phase of saiddelta-connected winding and coil groups of a next phase of saidstar-connected winding; wherein said coil groups are disposed withinseparate slots of said magnetic circuit so that in the cross section ofthe stator the phase zone occupied by each coil group is a multiple of30 within a range from 60 to 180 electrical degrees, with the exceptionof 150 electrical degrees.